The demands for wireless sensor networks will become more and more extensive with the development of informatization. However, sensor nodes in wireless sensor networks are generally miniaturized devices powered by batteries, their limited power limits their application in some special fields, such as some places that are difficult for human to reach, including ocean, desert and even nuclear radiation areas. Therefore, a new passive wireless sensor network becomes imminent.
Some researches have been carried out on such type of passive sensor network at home and abroad; for example, a method mainly used at abroad is to receive and modulate the energy broadcast by an aggregation node and then reflect the energy to the aggregation node through the ambient backscatter of the sensor node. While a method mainly used in china is to collect the weak energy emitted by the sensor node in the environment, and to transmit the information to the sink after the energy is accumulated to a sufficient amount. However, both methods have fatal defects. The first method simply modulates the incident wave and then reflects it back, which lacks sufficient routing structure and coding mode, and in order to achieve a satisfactory result, a high power requirement for a RF source will be needed. The second method needs an amount of time to enable the sensor node to accumulate enough energy, the method needs to collect the energy in the environment, and if the energy in the environment changes, the sensor node will be unstable.